Air supplying device

ABSTRACT

Improvement is made to the shape of blades of a fan assembly for sucking air into an annular wall through slits in the wall, in order to increase the aerodynamic performance and energy efficiency. In the fan assembly for sucking air in through slits provided in an annular wall ( 2 ), the blade tips of an axial fan ( 21 ) are bent in the rotating direction to smoothly take in air flowing in through the slits. The axial fan is formed so that the blades except the tips thereof are in a shape of a radial or a rearward tilting blade. The angle formed by the blade forward tilting angle near the blade tip of the axial fan and the slit angle is between −5 and 15°, and the blade tip is bent in the wind blowout direction. This configuration improves the P-Q characteristic, reduces noise and improves energy efficiency.

TECHNICAL FIELD

The present invention relates to a fan assembly used for electronicequipment and the like.

BACKGROUND ART

Due to the spread of small electronic equipment many attempts have beenmade in recent years to achieve high-density packaging of electriccircuits. Thus, due to the increase in the exothermic density ofelectronic equipment, axial or mixed-flow fans are used to cool theequipment.

In a conventional fan assembly, an axial fan 1 is placed in such amanner as to provide an appropriate space between blade tips of the fanand the inner circumferential surface of an annular wall 2, as shown inFIG. 20, so that in a blowing state in which a motor section 3 ispowered on, the axial fan 1 rotates around a shaft 4 to cause an airflow 5 from a suction side to a discharge side. In this blowing state,however, the speed of the air flow increases on the suction side of thetips of fan blades 8, and the energy of the air flow is converted into apressure energy. Consequently, inter-blade secondary flows occur at thetrailing edges of the blades to create low-energy areas at these edges.In this part of the fan assembly, a large loss is likely to occur torelease the flow, and in such a case, the air flow leaves a platesurface and vortexes occur in this area. As a result, turbulence noisemay increase to degrade the noise level and the static pressure-airquantity characteristic (hereinafter referred to as the “P-Qcharacteristic”). This phenomenon is frequently observed particularly ifthe discharge side is subjected to flow resistance (system impedance) tocause more leaking vortexes at the blade tips, thereby stalling theaxial fan.

In order to improve the characteristics of such an axial fan, the shapeof the annular wall provided at the outer circumference of the axial fanhas been improved in the fan assemblies described in Japanese PatentApplication No. 8-174042, Japanese Patent Application No. 9-151450 andJapanese Patent Application No. 9-260738 all assigned to the applicant.These fan assemblies are shown in FIGS. 21 to 23 wherein annular plates7 a to 7 e are provided in a casing body 9 as the annular wall 2encompassing the axial fan 1. The annular plates 7 a to 7 e arelaminated via spacers 13, and a slit 6 is formed between each pair ofadjacent annular plates 7 a to 7 e. In a blowing state, thisconfiguration allows air to be sucked into the annular wall 2 throughthe slits 6 provided between the annular plates 7 a to 7 e, in order torestrain occurrence of leaking vortexes at the blade tips as well asrotating stall, thereby improving the P-Q characteristic and reducingnoise. In addition, National Publication of International PatentApplication No. 6-508319 and U.S. Pat. No. 5,292,088 describe such fanassemblies that comprise a plurality of ring bodies arranged at theouter circumference of the axial fan at intervals so that air vortexesflowing through the gaps between the ring bodies increase the flow rateof the fluid. Alternatively, U.S. Pat. No. 5,407,324 describes a fanassembly wherein the inner circumferential portions of annular platesencompassing the outer circumference of the axial fan are inclined alongthe direction of the wind and wherein these annular plates areaccumulated so as to form a plurality of stages in order to enable airto flow between the inner and outer circumferences of the annular wall.

Although these inventions all improve the characteristics of the axialfan by sucking air from the outer circumference of the axial fan, theydescribe only the configuration of the ring bodies (annular plates)provided at the outer circumferential portion of the axial fan but donot particularly describe the shape of the axial fan. Thus, to make mostof the characteristics of the axial fan, the shape of the fan must beadjusted to the annular wall. The shape of the axial fan has beengenerally improved by cutting the blades of the axial fan in theircylindrical surfaces concentric with a rotating shaft of the axial fan,developing the cylindrical surfaces to be replaced by a planar infinitelinear blade series, applying to this blade series the linear bladeprofile series theory established for airplanes and the like in order topredict performance or to determine a three-dimensional shape suitablefor operating conditions.

FIGS. 24 to 29 show the shapes of conventional axial fans by way ofexamples. As shown in FIGS. 26 and 27, a cross section of a conventionalaxial fan 1 obtained by cutting it in a way of forming a cylinderconcentric with the rotating shaft is in such a form that wing-shapedblades 8 are joined together in the radial direction. This is becausethe air flows in the radial direction of the axial fan 1 are ignored indesigning the conventional axial fan. According to this design,calculated and actual values have not significantly deviated from eachother if the axial fan has an annular wall that prevents air fromflowing in from the outer circumference and if it is operated with arelatively low air flow resistance. In addition, in order to improve thecharacteristics of the axial fan when the air flow resistance isslightly high, an advancing blade is used in which the chord center lineof the blade is inclined at a specified angle in the rotating direction,as shown in FIGS. 28 and 29. In FIG. 24, a thin line h is aniso-thickness line denoting the thickness of the blade, an alternatelong and short dash line i is a chord center line obtained if the bladeis cut in a concentric cylindrical surface, and a broken line k denotesthe position of the maximum thickness obtained if the blade is cut in aconcentric cylindrical surface. When this conventional axial fan is usedin combination with the casing 9 with the slits provided in the annularwall therein, the air flows on the blades of the axial fan flow in thedirections shown by the arrows in FIG. 24. FIG. 25 shows the blade,which has been cut in the cross section shown by alternate long and twoshort line a—a′ that extends along this air flow. In FIG. 25, theneighborhood of the blade tip s is formed to be thicker to some degree,so air flows flowing onto this part collide against the surface of theblade tip and the air layer is released near both edges t1 of the tip.In addition, the distribution of the blade thickness, on which the bladeperformance significantly depends, substantially deviates from an idealblade shape arrangement, so the blade shape cannot be expected tocontribute to effecting a lift. The air layer is likely to be releasedat the trailing edge t2, thereby degrading the characteristics of theaxial fan.

An invention that does not suck air from the outer circumference of theannular wall but that attempts to improve the characteristics of theaxial fan by improving the shape of the blade tip is the impellerdescribed in Japanese Patent Laid-Open No. 6-307396 wherein theaerodynamic force is improved while noise is reduced by configuring thecross section of the outer circumferential blade tip so as to include asingle-side curved shape located at the leading edge and havingprojecting curves only on the pressure surface side; and a circularshape portion contiguous to the single-side curved shape. In addition,Japanese Patent Laid-Open No. 8-121391 describes an electric fan thatreduces aerodynamic noise by folding the outer circumference of theblade into a curve. Alternatively, Japanese Patent Application Laid-OpenNo. 8-284884 describes a fluid machine wherein the outside of the tip ofa moving blade is removed over a specified height from its tip end toform a thinner portion of a specified thickness at the inside of the tipin order to reduce the leakage of a fluid through the tip clearance,thereby improving the efficiency of an axial fan. It is premised thatthese conventional techniques for the shape of the axial fan, however,require to provide an annular wall preventing air from flowing in fromits outer circumference, so sufficient characteristics cannot beobtained by applying such blade shapes to a configuration for suckingair from the outer circumference of the annular wall, as describedabove.

An invention that requires air inflow through slits provided in theouter circumference of the axial fan in order to optimize the shape ofthe axial fan is the fan assembly in Japanese Patent Application No.9-260738 assigned to the applicant and shown in FIGS. 29 to 33. In FIG.30, a thin line h is an iso-thickness line denoting the thickness of ablade, an alternate long and short dash line i is a chord center lineobtained if the blade is cut in a concentric cylindrical surface, and abroken line k denotes the position of the maximum thickness in a crosssection obtained by cutting the blade at a concentric cylindricalsurface. FIG. 31 shows the blade, which has been cut in the crosssection shown by an alternate long and two short line a-a′ that extendsalong the air flow. As shown in FIG. 29, among sweepforward angles θ1 toθ3, the sweepforward angle θ3 at the blade tip is formed to be largerthan the two others. In other words, the blade is formed by folding theblade tips in the rotating direction. This configuration enables airflows flowing in through the slits to be smoothly taken in to improvethe P-Q characteristic of the fan assembly. Furthermore, the blade isshaped in such a way that as the blade tip approaches, the position ofthe maximum thickness in a cross section obtained by cutting the bladein a concentric cylindrical surface gradually moves backward toward thetrailing edge of the blade. Specifically, the cross sections of theblade along lines 1 ₁-1 ₁′, 1 ₂-1 ₂′, 1 ₃-1 ₃′, m-m′, and n-n′ shown inFIG. 32 are shaped as shown in FIGS. 33(a) to (e), respectively.Reference numeral F denotes the position of the maximum thickness. Asshown in FIG. 31, this shape maximizes the blade shape effect even onair flows flowing in from the outer circumference of the annular walland allows air flowing in through the slits to flow smoothly at theblade tip. Furthermore, according to this shape, the blade shape effectalso serves to cause a lift acting on air flows flowing in from theblade tip or the air layer is restrained from being released at thetrailing edge to enable the air flows flowing in through the slits to beeffectively converted into an air capacity, thereby further improvingthe P-Q characteristic of the fan assembly.

An object of the present invention is to further improve the blade shapeof the fan assembly that sucks air into the annular wall through theslits provided in these walls as in Japanese Patent Application No.9-260738, thereby improving the aerodynamic performance or energyefficiency.

DISCLOSURE OF THE INVENTION

In a fan assembly according to the present invention, to attain theabove object, an annular wall is formed with a space left from the bladetips of a fan, and a plurality of slits communicating the inner andouter circumferential portions of the annular wall with each other areformed in a portion of the annular wall opposite to the blade tips ofthe fan. The fan is formed in a radial blade shape with a zerosweepforward angle, wherein the blade tips are bent in the rotatingdirection while the portions of the blades other than the tips thereofare not inclined in the rotating direction.

In addition, in the fan assembly according to this invention, an annularwall is formed with a space left from the blade tips of a fan, and aplurality of slits communicating the inner and outer circumferentialportions of the annular wall with each other are formed in a portion ofthe annular wall opposite to the blade tips of the fan. The fan isformed so that a blade has a rearward projecting angle in which theblade tips are bent in the rotating direction while other portionsthereof than the blade tips are inclined in the direction opposite tothe rotating direction.

In addition, in the fan assembly according to this invention, an annularwall is formed with a space left from the blade tips of a fan, and aplurality of slits communicating the inner and outer circumferentialportions of the annular wall with each other are formed in a portion ofthe annular wall opposite to the blade tips of the fan. The fan isconfigured so that the blade tips are bent in the rotating direction,the forward tilting angle of the blade tip relative to the radialdirection is −5 to 15°, and the blade tip and the vicinity thereof arebent in the wind blowout direction.

In addition, one of these fan assemblies is provided in a electronicequipment as a fanning means.

The above configuration allows air flowing in through the slits to besmoothly taken in, thereby improving the P-Q characteristic of the fanassembly and reducing noise from the fan assembly. In addition, if theabove fan assembly is provided in a electronic equipment such as apersonal computer, noise from the electronic equipment can be reducedand the cooling and energy efficiency can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view showing an axial fan assembly according to anembodiment of this invention;

FIG. 2 is a side view showing the axial fan assembly;

FIG. 3 is a sectional view showing the axial fan assembly;

FIG. 4 is a front view of an axial fan of a general forward-tiltingblade type;

FIG. 5 is a front view of an axial fan of a general radial blade type;

FIG. 6 is a front view of an axial fan of a general rearward-tiltingblade type;

FIG. 7 is a blade iso-thickness diagram of an axial fan according to anembodiment of this invention;

FIG. 8 is a sectional view of the axial fan according to the embodiment;

FIG. 9 is a front view of the axial fan according to the embodiment;

FIGS. 10(a) to (e) are sectional views showing the thickness of eachsection of a blade of the axial fan in FIG. 9;

FIG. 11 is a front view of an axial fan showing another example of theembodiment;

FIG. 12 is a front view of a fan assembly according to anotherembodiment of this invention;

FIGS. 13a-c are sectional views of each blade chord of the fan assemblytaken along lines a-a′, b-b′ and c-c′ of FIG. 12, respectively.

FIG. 14 is a front view of a fan assembly according to another exampleof the embodiment in FIG. 12;

FIGS. 15(a) to (c) are sectional views obtained by cutting the fanassembly in FIG. 14 through each blade chord center line in the axiallongitudinal direction;

FIG. 16 is an explanatory drawing for describing a blade theory;

FIG. 17 is an explanatory drawing for describing the blade theory;

FIG. 18 is a front view of a conventional fan assembly;

FIGS. 19(a) to (c) are sectional views of each blade chord of the fanassembly in FIG. 18;

FIG. 20 is a sectional view showing a conventional fan assembly;

FIG. 21 is a front view showing a slitted fan assembly according to aprior art;

FIG. 22 is a sectional view showing the slitted fan assembly accordingto the prior art;

FIG. 23 is a sectional view showing the slitted fan assembly accordingto the prior art;

FIG. 24 is a blade iso-thickness diagram of a conventional axial fan;

FIG. 25 is a sectional view of the conventional fan;

FIG. 26 is a front view of the conventional axial fan;

FIGS. 27(a) to (c) are sectional views showing the thickness of eachportion of a blade of the conventional axial fan;

FIG. 28 is an explanatory drawing of a conventional blade shape;

FIG. 29 is an explanatory drawing of a blade shape according to theprior art;

FIG. 30 is a blade iso-thickness diagram of an axial fan according tothe prior art;

FIG. 31 is a sectional view of the axial fan according to the prior art;

FIG. 32 is a front view of the axial fan according to the prior art; and

FIGS. 33(a) to (e) are sectional views showing the thickness of eachportion of a blade of the axial fan according to the prior art.

EMBODIMENTS OF THE INVENTION

An embodiment of this invention will be described below with referenceto the drawings. FIGS. 1 to 3 show a fan assembly according to thisembodiment. Members similar to those shown above have the same referencenumerals, and their description is omitted. As shown in FIG. 2, thewidth W of laminated annular plates 7 a to 7 e is set at the same valueas the axial width of an axial fan 21 or almost the same value as theaxial width of an axial fan 1. In addition, the width w of the gapbetween slits 6 is continuously varied so as to almost equalize theinflow resistance of each portion. When the axial fan 1 is rotationallydriven, a negative pressure is generated on the suction side of the tipsof blades 28, and due to the difference between this pressure and theatmospheric pressure outside the slits 6, air flows 5 s flow toward theinterior through the slits 6. When the width w of the gap between theslits 6 is set at an appropriate value, the air flows 5 s flowingthrough the slits 6 become layer flows to restrain leaking vortexesgenerated at the blade tips and flowing from a positive pressure side toa suction side. This configuration prevents the air flows from leavingthe suction surface to improve the P-Q characteristic while reducingnoise.

Although the axial fan is generally molded by means of resin injection,injection molding limits the shape of the axial fan due to theconfiguration of molds and axial fans of an advancing blade type formedby means of injection molding disadvantageously have a small blade axialprojected area. FIG. 4 shows an axial fan of a blade type having aforward tilting angle in which the chord-wise central position of theblade is inclined in the rotating direction (its sweepforward angle hasa positive value), FIG. 5 shows an axial fan of a radial blade type inwhich the chord-wise central position of the blade is on the radius (itssweepforward angle is zero), and FIG. 6 shows an axial fan of a bladetype having a rearward projecting angle in which the chord-wise centralposition of the blade is inclined in the direction opposite to therotating direction (its sweepforward angle has a negative value). In allcases, the outer diameter of the blade is the same. The size c of thegap between the adjacent blades is restricted by the structure of themold and must be constant for any shape. As shown in FIGS. 4 to 6, ifthe sizes c of the gaps between the adjacent blades are set equal, theaxial fans of such blade types as having a forward tilting angle and arearward projecting angle have a smaller blade axial projected area thanthe axial fan of a radial blade type and fail to provide the sameperformance as the radial blade type unless the workload of the bladeper area is increased. Increasing the workload of the blade requires theblade angle (the torsion angle of the blade around the radial shaft) tobe increased, but increasing the blade angle may increase the airresistance of the blade and thus the axial-fan driving force and mayrelease the boundary layer on the blade suction side earlier, frequentlyresulting in stalling.

Thus, this embodiment optimizes the shape of the blade tip based on theaxial fan of the radial blade type having the smallest workload of theblade per area, that is, the smallest blade load. FIGS. 7 to 10 show anaxial fan 21 according to this embodiment. In FIGS. 7 to 10, the shapeof the tip of a blade 28 is almost the same as that of the axial fan inJapanese Patent Application No. 9-260738 shown in FIGS. 29 to 33, butthis blade differs from that in Japanese Patent Application No. 9-260738in that except for the tip, the blade is shaped like a radial bladehaving a zero sweepforward angle to provide a larger blade axialprojected area despite the same size of the axial fan.

The shape of the axial fan 21 will be described in detail and clarifiedbelow. In FIG. 7, the blade tip s of the axial fan 21 is formed byfolding it in the rotating direction. Air flows flowing in through theslits 6 form flows v advancing in an almost radial direction, and theblade tip is rotated at a peripheral speed u. Thus, relative air flowsflow in from a direction w as seen from the blade 28. Folding the bladetip in the rotating direction smoothes these air flows. To equalize thiswind flow with the forward tilting angle of the blade tip of the axialfan, the sweepforward angle θ3 at the blade tip is preferably set so asto meet the following condition.

θ=tan⁻¹ (u/v)

This setting allows the wind to flow in most smoothly and providesadvantageous conditions in terms of both the P-Q characteristic andnoise. In addition, in FIG. 7, thin line h is an iso-thickness linedenoting the thickness of the blade 28, alternate long and short dashline i is a chord center line in a cross section obtained if the blade28 is cut in a concentric cylindrical surface, and broken line k denotesthe position of the maximum thickness in a cross section obtained if theblade 28 is cut in a concentric cylindrical surface. FIG. 8 shows theblade 28, which has been cut in the cross section shown by alternatelong and two short line a-a′ that extends along the air flow.Furthermore, the cross sections of the blade 28 along the lines 1 ₁-1₁′, 1 ₂-1 ₂′, 1 ₃-1 ₃′, m-m′, and n-n′ shown in FIG. 9 are shaped asshown in FIGS. 10(a) to (e), respectively. Reference numeral F denotesthe position of the maximum thickness. As shown in FIG. 10, the blade isshaped in such a way that as the blade tip approaches, the bladethickness decreases while the position F of the maximum thicknessgradually moves backward toward the trailing edge of the blade. As shownin FIG. 8, this shape maximizes the blade shape effect even on air flowsflowing in from the outer circumference of the annular wall and allowsair flowing in through the slits 6 to flow smoothly at the blade tips.Furthermore, according to this shape, the blade shape effect also servesto cause a lift acting on air flows flowing in from the blade tip or theair layer is restrained from being released at the trailing edge toenable the air flows flowing in through the slits 6 to be effectivelyconverted into an air capacity, thereby further improving the P-Qcharacteristic of the fan assembly.

Furthermore, in the axial fan 21 according to this invention, the bladeis shaped into a radial blade type except for its tip, so it has a largeaxial projected area of the blade 28 and provides as high performance asin the prior art despite the small workload of the blade 28 per area.Besides, due to its ability to reduce the blade angle of the blade 28,this invention can provide a fan assembly that can reduce the drivingforce required for the blades 28 while restraining stalling caused bythe early release of the boundary layer on the blade suction side andthat thus has a high blowing ability compared to the required drivingforce, in other words, has a high energy efficiency. In addition, if theaxial fan 21 is driven by a motor, both the power consumption andheating of the motor can be restrained to improve the cooling efficiencyof equipment incorporating this fan assembly.

If the shape of the blade tips of the axial fan of the blade type havinga rearward projecting angle is optimized using the same conditions asdescribed above, as shown in FIG. 11, and when the fan assembly isoperated under a certain blowing resistance, the pressure distributionon the blade surface causes the air flows on the blade suction surfaceto flow in directions slightly inclined toward the inner circumferenceas shown by the arrows in the figure. In this case, the air flows on theblade suction surface flow over the shortest distance to reduce the flowvelocity on the suction surface where the boundary layer is likely to bereleased, so the blade angle can be increased correspondingly withoutcausing the boundary layer to be released, thereby increasing the bladeangle from the blade tip to a boss section to allow even a blade shapenear the boss to work, the blade shape being conventionally engaged inlittle work. Consequently, although the effect of improving the energyefficiency cannot be expected, this embodiment can provide a fanassembly of a large air capacity. Alternatively, a small fan assembly ofa large air capacity can be provided that restrains the boundary layerfrom being released to allow the axial fan to rotate at a high speedeven under operating conditions such as the fast rotation of the axialfan that are likely to cause the release of the boundary layer.

Next, another embodiment of this invention will be explained. Memberssimilar to those shown above have the same reference numerals, and theirdescription is omitted. Although the above first embodiment optimizesthe shape of the axial fan by mainly focusing on the projection of theaxial fan in the axial direction, this second embodiment focuses onsectional shapes obtained by cutting the axial fan along each chord.

FIGS. 18 and 19 show the fan assembly in Japanese Patent Application No.9-260738 shown in FIGS. 29 to 33. In the sectional shape obtained bycutting the axial fan of this fan assembly along each chord, the leadingedge, middle, and trailing edge of the blade all extend almostperpendicularly to the shaft, and the forward tilting angle of the bladetip is set equal to the slit angle, as shown in FIGS. 19(a), (b), and(c). This configuration allows components of the wind flowing along thissectional direction to be smoothly introduced, while precluding theaxial fan from working for these components,

FIG. 12 shows a fan assembly according to this embodiment. The sectionalshapes obtained by cutting an axial fan 31 of this fan assembly alongeach chord differ from those in the above embodiment in that a blade 38configures a forward tilting blade in which the blade tip direction isinclined toward the wind suction side and in that the blade tip isslightly inclined forward and toward the wind suction side relative tothe angle of the slit 6, as shown in FIGS. 13(a), (b), and (c). Theforward tilting angle of the blade tip is smaller than that of the otherportions so that the blade tip is bent in the wind blowout direction.The reason for the use of the different forward tilting angle for theblade 38 will be described in light of the blade theory. FIG. 16 shows atwo-dimensional blade that is cambered. In FIG. 16, angle j is referredto as an incidence angle formed by the camber line at the blade leadingedge and the wind inflow direction. FIG. 17 shows the relationshipbetween the lift and drag generated when the wind incidence angle j ofthis blade is varied. The blade performance is improved as the liftincreases or the drag decreases, but the incidence angle that maximizesthe lift acting on the blade is different from the incidence angle thatminimizes the drag (air resistance) acting on the blade, as shown inFIG. 17. In general, despite the dependence on the shape of the blade,the condition for maximizing the lift is a positive incidence anglebetween 5 and 15°, and the condition for minimizing the drag is anincidence angle close to zero, that is, between −5 and 5°.

When the above blade theory is applied to the flows along the crosssections of the axial fan 31 according to this embodiment obtained bycutting the fan 31 along each chord, the incidence angle can be assumedto be the angle j (shown in FIG. 13) formed by the slit 6 angle and theforward tilting angle of the blade tip. If the blade tip has a certainincidence angle and the condition for increasing the lift isestablished, that is, the blade is shaped to have a forward tiltingangle, those components of the wind sucked in through the slits 6 whichflow in the sectional direction can be effectively converted into an aircapacity to increase the existing air capacity. In addition, by settingthe angle formed by the forward tilting angle and the slit 6, at a valueclose to zero to reduce the drag acting on the blade tip, the energyloss in this portion can be reduced to increase the energy efficiency ofthe entire axial fan. The blade according to this embodiment focuses onthe air capacity by providing a certain angle between the forwardtilting angle of the blade tip and the slit 6. In general, in order toprovide such a characteristic, the angle between the forward tiltingangle of the blade tip and the slit 6 must be between −5 and 15° and thetip must be bent in the wind blowout direction With too large an anglebetween the forward tilting angle of the blade tip and the slit 6 angle,the boundary layer may be released on the suction side of the blade 38to reduce the efficiency and the air capacity. With too small an angle,the lift generation is prevented to reduce the air capacity, therebyreleasing the boundary layer on the positive pressure side of the blade38 to reduce the efficiency. In addition, if the tip of the blade 38 isbent in the wind suction direction, the blade tip has the oppositecamber direction to cause a lift acting in the opposite direction,thereby reducing the air capacity. In addition, although in thisembodiment, the forward tilting angle of the blade is almost constantexcept for the blade tip, this configuration increases the axial lengthof the axial fan 31 and thus the size of the fan assembly in thedirection of the fan shaft. Thus, the neighborhood of the tip of a blade48 of the axial fan 41 is bent in the wind blowout direction whereas theroot of the blade is bent in the wind suction direction so that thecross section of the blade 48 is S-shaped, as shown in FIGS. 14 and 15.Then, air flows flowing in from the blade 48 tip flow out from the bladetrailing edge before reaching the blade root, as shown in FIG. 11, sothe air flows near the blade root move almost along the circumference.Accordingly, a fan assembly that provides the maximum P-Q characteristiccan be provided by bending the blade tip in the wind blowout direction,while bending in the wind suction direction the neighborhood of theblade 48 root, which is not significantly affected by radial flows, toreduce the length of the axial fan 41 in the direction of the fan shaftand thus the size of the fan assembly, in particular, the axial size.

Although this embodiment has shown the blade type having a forwardtilting angle as the shape of the axial fan, similar effects can beobtained by applying this embodiment to the axial fan of the radial orthe blade type having a rearward projecting angle shown in the aboveembodiments. Due to their synergetic effect, this combination canimprove the energy efficiency or further improve the P-Q characteristic.If the above fan assembly is provided in electronic equipment, forexample, a personal computer, noise from the electronic equipment can bereduced and the cooling and energy efficiency can be improved.

As described above, this invention forms the plurality of slits makingthe inner and outer circumferential portions of the annular wall incommunication with each other and bends the tips of the blades of thefan in the rotating direction. This configuration enables air flowsflowing in through the slits to be smoothly taken, thereby improving theP-Q characteristic of the fan assembly and reducing noise from the fanassembly. Furthermore, it can improve the energy efficiency of the fanassembly.

What is claimed is:
 1. A fan assembly comprising an annular wall (2)formed with a space left from blade tips of a fan, and a plurality ofslits (6) formed in a portion of the annular wall (2) opposite to saidblade tips of the fan, said slits communicating the inner and outercircumferential portions of the annular wall (2) with each other,wherein said fan is formed to have a radial blade shape with a zerosweepforward angle, in which the tip of a blade (28) is bent in therotating direction while the other portion of the blade than the tipthereof is not inclined in the rotating direction.
 2. The fan assemblyaccording to claim 1 wherein the fan is configured so that a crosssection obtained by cutting it in a concentric cylindrical surface of arotating shaft is in a blade shape and in each cross section, a positionof the maximum thickness of the blade moves backward toward a bladetrailing edge according as the position approaches the blade tip.
 3. Thefan assembly according to claim 1 wherein the fan is configured so thatthe sweepforward angle θ at the blade tip, the average flow velocity vof air flowing in from the outer circumferential direction of theannular wall, and the peripheral speed u at the blade tip satisfy thefollowing equation: θ=tan⁻¹ (u/v).
 4. A fan assembly comprising anannular wall (2) formed with a space left from blade tips of a fan, anda plurality of slits (6) formed in a portion of the annular wall (2)opposite to said blade tips of the fan, said slits communicating theinner and outer circumferential portions of the annular wall (2) witheach other, wherein said fan is formed to have a rearward projectingblade shape, in which the tip of a blade (28) is bent in the rotatingdirection and the other portion of the blade than the tip thereof isinclined in the direction opposite to the rotating direction.
 5. A fanassembly comprising an annular wall (2) formed with a space left fromblade tips of a fan, and a plurality of slits (6) formed in a portion ofthe annular wall (2) opposite to said blade tips of the fan, said slitscommunicating the inner and outer circumferential portions of theannular wall (2) with each other, wherein said fan is configured so thatthe tips of the blades (28, 38, 48) are bent in the rotating direction,the sweepforward angle of the blade tips relative to the radialdirection is −5 to 15°, and the blade tips and the vicinity thereof arebent in the wind blowout direction.
 6. The fan assembly according toclaim 5 wherein the fan is configured so that the cross section of theblade obtained by cutting it along the center line of each blade chordin the axial longitudinal direction is curved in an S-shape.
 7. The fanassembly according to claim 5 wherein the fan is formed so that theblades except the tips thereof are in a shape of a radial or a rearwardtilting blade.
 8. A fan assembly comprising an annular wall (2) spacedfrom tips of blades of a fan, and a plurality of slits (6) formed in aportion of the annular wall (2) opposite said blade tips, said slitscommunicating the inner and outer circumferential portions of theannular wall (2) with each other, wherein said fan blades extendingradially with a zero sweepforward angle, in which the tip of a blade(28) is bent in the rotating direction while the portion of the bladeother than the tip thereof is not inclined in the rotating direction,and wherein the thickness of the blade near the blade tip decreasestoward the blade tip.
 9. Electronic equipment including a fan assemblycomprising an annular wall (2) spaced from tips of blades of a fan, anda plurality of slits (6) formed in a portion of the annular wall (2)opposite to said blade tips, said slits communicating the inner andouter circumferential portions of the annular wall (2) with each other,wherein said fan blades extend radially with a zero sweepforward angle,in which the tip of a blade (28) is bent in the rotating direction whilethe portion of the blade other than the tip thereof is not inclined inthe rotating direction.